1. Technical Field
The present invention relates to an image display device for displaying images with a great number of gradation steps, and more particularly, to an image display device and a projector highly suitable for displaying images with high gradation.
2. Related Art
In recent years, image quality of electronic display devices such as an LCD (Liquid Crystal Display), an EL (Electro-luminescence) display, a CRT (Cathode Ray Tube), and a projection type display has been remarkably improved. A device having characteristics such as resolution and color gamut almost comparable to a visual characteristic of human eyes has been put into practice.
However, a reproduction range of a luminance dynamic range is limited to about 1 to 102 [nit], and the gradation step is generally expressed by 8 bits.
On the other hand, it is said that human eyes can perceive a luminance dynamic range of about 10−2 to 104 [nit] at a time and has a luminance differentiation capability of 0.2 [nit]. When the luminance dynamic range is converted into a gradation number in consideration of the luminance differentiation capability, it can be said that an amount of data corresponding to around 12 bits are required.
When a display image of a current electronic display device is viewed through such visual characteristic, the luminance dynamic range is conspicuously narrow. Moreover, since the gradation resolution in the shadow portions and the highlight portions is insufficient, the reality of the display image and its power of expression seem inadequate.
Furthermore, images in computer graphics (CG) used in movies, games and the like, there is a mainstream trend to pursue reality of depiction by giving the display data with the luminance dynamic range and gradation characteristic close to those of the human vision.
However, since the electronic display device has insufficient capability, there is a problem that the power of expression inherent to CG contents (which has a greater number of bits for expressing the gradation) is not fully exerted when displaying images of the CG contents.
Further, in the next-generation Windows®, the adaptation of a 16-bit color space is planned, and the dynamic range and the gradation steps will increase remarkably in comparison with the current 8-bit color space. Therefore, the need for an electronic display device capable of fully expressing the 16-bit color space and achieving a high dynamic range and high gradation is expected to increase.
Among the electronic display devices, a project ion type display device (projector), such as a liquid crystal projector and a DLP (registered trademark) (Digital Light Processing) projector, can display an image on a large screen, and is therefore effective for reproducing the reality and the power of expression of displayed image.
In the electronic display device, various proposals have been suggested in order to widen the luminance dynamic range.
For example, in the case of using a digital-driven modulation display element such as a ferroelectric liquid crystal panel or a reflection type optical modulation element (DMD), a subfield driven time integral gradation scheme is used as a method of displaying the gradation steps, and the entire pixels are rewritten in the unit of a plane, i.e., the entire pixels are simultaneously turned on or off in the unit of a display screen (see JP-A-2000-259126 for example).
On the other hand, in the case of using a liquid crystal display panel, a display process is performed in the unit of scanlines in which each pixel is driven in a line-sequential manner (see JP-A-2001-125067 for example).
However, in the case of the image display device with the two-modulation optical system configured by the use of the digital-driven modulation display element and the liquid crystal display element, the image data of the digital-driven modulation display element are switched in the unit of planes whereas the image data of the liquid crystal display element are switched in the unit of lines. The gradation setting values may deviate greatly with time between the first scanline and the last scanline on the screen at the time of switching the image data in the unit of planes. Accordingly, it is difficult to improve contrast ratio on the entire display screen.
In addition, in the liquid crystal display element, due to an orientation change response characteristic, it takes time for a luminance value of output light beams to become a value corresponding to a control voltage after application of the control voltage, thereby causing a problem of further increasing deviation of the setting times.
When the two-modulation optical system is configured by the combination of the digital-driven modulation display element of which the entire image data are switched in the unit of planes and the liquid crystal display element in which the image data are switched in the unit of scanlines, timings for displaying gradation become inconsistent with each other between the digital-driven modulation display element and the liquid crystal display element, which makes it difficult to display an exact gradation characteristic that alms to realize a high contrast ratio, thereby deteriorating an image quality.
In addition, JP-A-2000-259126 discloses a technology in which, in order to improve a moving picture display performance of the liquid crystal display element, black display timing is inserted at every time of displaying a frame by controlling lighting or un-lighting of the light source so as to absorb the timing deviation, thereby improving the moving picture display performance.
According to the technology disclosed in JP-A-2000-259126, an individual lighting control is performed in which backlights are divided in correspondence with scanlines at a predetermined area and the back light corresponding to areas of the scanlines to which the image data are rewritten is sequentially lighted in consideration of the line-sequential driving of the liquid crystal display element and the response characteristic of the liquid crystal.
In the technology disclosed in JP-A-2000-259126, without the individual controlling, it is difficult to obtain a desired gradation display since decrease in the display luminance becomes prominent due to the orientation change of the liquid crystal, i.e., a transient characteristic of rising and falling of the liquid crystal.
However, in the technology described above, it requires a special areal light source or a driving control circuit for lighting the light source at each of the areas, thereby increasing the production cost.
Although it is controlled individually, assuming that a cathode ray tube used in the liquid crystal display element or the like is generally used as the light source, it may be difficult to control the lighting to a delicate unit (the number of scanlines), which makes it difficult to prevent the deterioration of the image quality.